POSTNATAL-DEVELOPMENT OF METHIONINE-ENKEPHALIN MODULATION OF CHOLINERGIC TRANSMISSION IN CAT ILEUM

The age-dependent effects of methionine-enkephalin (Met-enk) on contra ctions and [H-3]-acetylcholine ([H-3]-ACh) release evoked by electrica l field stimulation (EFS) were studied in vitro on cat terminal ileum of young (2-month old) and adult cats. Radioactivity was measured by l iquid scintillation spectrometry and the effect of Met-enk was evaluat ed by the FrS2/FrS1 ratio. Scopolamine, 1 mu M, abolished the EFS-indu ced contractions. Met-enk dose dependently (0.01-1 mu M) decreased the EFS-induced contractions. The inhibitory effect of Met-enk on the evo ked contractions was more pronounced in adult cats compared to that in young cats. In adult animals Met-enk, 1 mu M, almost completely inhib ited the evoked contractions (94.6 +/- 2.6% of inhibition) while in th e young cats its inhibitory effect was less pronounced (52.1 +/- 6.8%) . Met-enk decreased the EFS-evoked release of [H-3]-ACh by 44.6 +/- 4. 6% in-adult cats and by 21.6 +/- 1.6% in young cats. The data suggest opioid modulation of cholinergic transmission in cat terminal ileum at neuronal level occuring as early as the postnatal period and increasi ng with the ontogenesis.